Trickle chargers constantly apply a small current to charge batteries. Inexpensive.
Timer based chargers are trickle chargers with a timer to protect the batteries from overcharge. Inexpensive.
Smart chargers use a microchip to end the charge when the batteries are full. For AA and AAA. Protects your investment.
Universal chargers can handle a wide variety of batteries; AA, AAA, C, D and 9V. Universal chargers are available in both Timer and Smart versions. We always recommend smart chargers as they maximize your battery investment.